(1) Field of the Invention
The present invention relates to an ink recording apparatus for use in printers or the like. It is to be noted that the word `recording` herein used refers to the fact that any desired patterns of characters, symbols, or the like are written down onto a printed material such as paper with ink jetted out by an apparatus of the present invention.
(2) Description of the Related Art
In conventional ink recording apparatus that are currently used in printers featuring their compactness suitable for office or personal use thereof, two types of ink jet systems described below are mainly employed. One of the types is a piezoelectric type in which the ink is applied by piezoelectric elements within a ink chamber. Another type is a thermal type in which the ink generates bubbles by the utilization of exoergic of heating elements within a ink chamber.
The piezoelectric type controls the diameter of ink droplets well. However, to arrange the piezoelectric elements in which density is so difficult that the ink jet head is forced to become large in size. Accordingly, it is impossible for an ink jet head to discharge many ink droplets simultaneously. Then, the ink jet head is required to move for recording many dots. The result is that the ink jet head takes a long time to record a predetermined size image from the overall point of view of the recording speed, even if the frequency of ink droplet discharge can be increased within a feasible range. And also, a moving mechanism for the ink jet head is required, which prevents the ink recording apparatus from increasing its compact and lightweight performance, it quiet performance and decreasing demanded power and cost.
On the other hand, the recording speed of thermal type can be increased enough, because a line head can be manufactured easily by substantially integrating the heating elements. However, it needs an independent ink chamber and a nozzle for each heating element. The thermal type ink recording apparatus having such a structure requires a high work precision in the manufacture thereof. The work precision and positional precision between a substrate provided with the thermal elements and the ink chambers have an effect on the ink jet amount so that, not only the control of the ink droplet diameter, but also the manufacturing of the head becomes difficult resulting in a higher cost. Moreover, since the integrating yield of heating elements does not reach the practical level, its cost actually becomes enormous.
A conventional ink recording apparatus is shown in the Japanese magazine "Nikkei Mechanical", issued on May 29, 1989, pp. 90 to 91, the apparatus exemplifying such ink recording apparatus that solve such problems as described above.
FIG. 14 shows a construction of such a conventional ink recording apparatus. In the figure, a slit plate 1 is provided with a plurality of slits 2 having a width of 50 .mu.m and a length of 8 mm in place of nozzles. The slit plate 1 has also a plurality of auxiliary holes 3 equal in number to a plurality of heating elements 5 formed on a base plate 4, with an ink reservoir 6 as well provided on the slit plate. On the base plate 4 there are formed a plurality of electrodes 7 in correspondence with the heating elements 5 and, moreover, a plurality of fluid resistance elements 8 shaped into long, narrow protrusions. Between the slit plate 1 and the base plate 4 there is disposed a spacer 9, which in conjunction with the slit plate 1 and the base plate 4 defines a portion serving as an ink chamber 11 illustrated in FIGS. 15a to 15d. Under the base plate 4 there is provided an ink tank 10, whereon all the units are piled up to make up a head. The heating elements 5 are formed by piling up a glass layer, resistors, electrodes, and a protective coat on the base plate 4, as in a common thermal head.
A conventional ink recording apparatus having a construction as described above will jet ink droplets while taking the steps as shown in FIGS. 15a to 15d. Each step is detailed below:
(a) First, when a pulse voltage is applied to the heating elements 5 on the base plate 4 to heat the ink contained in the ink chamber 11, the ink in the vicinity of the heating elements 5 vaporizes to make a large number of small bubbles 12;
(b) Second, the small bubbles 12 merge together and grow into a larger bubble 13 that overcome the surface tension, causing ink swells to be produced at the slits 2;
(c) Third, when the heating elements 5, on completion of heating, are cooled down to stop the bubble 13 from being produced, the swelling of ink is intercepted to produce ink droplets 14; and
(d) Finally, the ink droplets 14 are jetted out through the slits 2 by the power of growing bubble 13.
If a number of heating elements 5 share the slits 2 and the ink chamber 11 with one another as in the above conventional apparatus, there arises a problem that the ink droplets 14 derived from adjoining heating elements 5 may interfere with each other. In the conventional apparatus, however, the fluid resistance elements 8 provided between adjoining heating elements 5, as shown in FIG. 14, will serve to prevent pressure waves from being horizontally propagated while the bubbles are being produced, thereby allowing the ink droplets 14 to be formed and jetted out without being adversely affected by such pressure waves. Furthermore, the auxiliary holes 3 provided in the slit plate 1 will absorb the pressure waves, so that pressure waves may be prevented also from being reflected.
In the conventional apparatus arranged as described above, however, the heating elements 5 must eventually be used up in the apparatus, which does not lead the cost of the ink jet head toward any substantial reduction. And since the heating elements 5 need cooling, the recording speed is not increased substantially. Further, during the alternation between heating and cooling under the condition in which the heating elements 5 are wetted with ink, the burnt ink is caused to adhere to the surface of the heating elements 5. The growth of the burnt ink gradually changes the initial ink jet performance, and an inferior recording finally occurs. When the adhesion of burnt ink, in which a coloring matter like an organic dye and thermally decomposed carbon of different types of additives are mainly included, progresses on the surfaces of the heating elements 5, not only the bubbles 12 are generated inhomogeneously, but also the heating elements 5 are thermomechanically fatigued and finally destroyed. Moreover, when the burnt ink is suspended in the ink, it will stop up the slits 2, preventing the ink droplets 14 from being jetted out therethrough.
With respect to the ink properties, in order for the ink to generate the pressure and transmit the pressure to the slits 2, the ink is preferably of a type easy to evaporate and also low enough in viscosity to transmit the pressure consistently with minimal pressure loss. On the other hand, in order for the ink droplets 14 jetted through the slits 2 to fix on the recording paper (not shown), the ink is preferably of a type hard to dry to avoid any possible clogging in the slits 2 and high enough in viscosity to stabilize the jet of ink travelling toward the recording paper and also to avoid any possible running of ink. Particularly, a cheap recording paper generally has a rough surface, since the running of ink can be remarkable on such a surface, the ink is required to have a high viscosity and fixability to allow free selection of recording paper. Thus, the ink is required to have two properties which conflict with each other. It is impossible to design an ink that satisfies the two conflicting requirements. Accordingly, the ink recording apparatus of the prior art is forced to sacrifice at least one of the three performance elements, that is, the lifetime of the heating elements 5, the generating sensitivity of the bubbles 12 and the recording quality. Further, the apparatus needs its own recording paper on which the ink is hard to run. These factors increase not only initial cost but also running cost of the apparatus.